Hydromechanical transmission with differential steer

ABSTRACT

A transmission has an HMT which is in parallel with an HST both driving two planetaries which are used for forward/reverse and for differential steer. A two or three mode HMT is created by having a first HST in parallel with two or three mechanical power paths defined by separate clutches. One clutch has a speed reversing gear to produce reverse output speed. A four-element planetary sums the parallel flow and delivers variable speed and torque to two output shafts. The differential steer is created by two planetaries connected with the outputs of the HMT and a second HST. The planetaries have a speed reversing gear on one power path connection. The second HST controls the differential speed between the output shafts by adding speed to one and subtracting speed from the other.

BACKGROUND OF THE INVENTION

[0001] There are a number of skid steer vehicles that need to havetransmissions which have the capability to provide a separate controlledspeed output to each side of the vehicle in order to steer it. Theseinclude skid steer loaders, crawler tractors and loaders, tracked farmtractors, asphalt pavers and utility machines. These vehicles may havewheels or tracks, and if wheeled may have either a fixed or variablewheel geometry. Many of these vehicles have a hydrostatic transmissionfor each side of the vehicle with a separate speed control for eachtransmission in order to steer, typically referred to as a dual pathtransmission. These dual path transmissions must be coordinated in orderto achieve both steering and forward or reverse motion control.

[0002] In order to increase the utility of these vehicles, output speedsin the forward direction of travel are increasing. As wheeled vehiclessuch as skid steer loaders have a short wheelbase, the need for precisecontrol of the steer function increases as speed increases. Dual pathtransmissions may not provide the necessary control for these higherspeed vehicles.

[0003] High efficiency of operation is also becoming more important inorder to reduce operating cost. Compact size is important for ease ofinstallation.

[0004] It is therefore a principal object of this invention to provide ahydromechanical transmission with differential steer which accommodatesthe need for increasing vehicle speeds with good steer control,particularly in wheeled vehicles such as skid steer loaders.

[0005] A further object of the invention is to provide a hydromechanicaltransmission with differential steer which satisfies the needs for highefficiency, compact size and low cost.

[0006] These and other objects will be apparent to those skilled in theart.

SUMMARY OF THE INVENTION

[0007] A transmission has an HMT which is in parallel with an HST, bothdriving two planetaries which are used for forward/reverse and fordifferential steer. A two or three mode HMT is created by having a firstHST in parallel with two or three mechanical power paths defined byseparate clutches. One clutch has a speed reversing gear to producereverse output speed. A four-element planetary sums the parallel flowand delivers variable speed and torque to two output shafts. Thedifferential steer is created by two planetaries connected with theoutputs of the HMT and a second HST. The planetaries have a speedreversing gear on one power path connection. The second HST controls thedifferential speed between the output shafts by adding speed to one andsubtracting speed from the other.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a schematic plan view of a skid loader with a transverseengine;

[0009]FIG. 2 is a schematic plan view of a skid loader with alongitudinally disposed engine;

[0010]FIGS. 3A and 3B are graphs showing transmission average outputspeed vs output torque, respectively, vs. HST F-unit speed for a 2 modeand 3 mode HMT;

[0011]FIG. 4 is a schematic drawing of HMT circuitry and componentshaving coaxial clutches and series steer planataries;

[0012]FIG. 5 is a block diagram for the transmission of FIG. 4;

[0013]FIGS. 6A and 6B are schematic drawings of a planetary gear inelevation and in section, respectively;

[0014]FIGS. 7A and 7B are views similar to those of FIGS. 6A and 6B fora different planetary;

[0015]FIG. 8 is a schematic drawing of HMT circuitry and componentshaving parallel axis planataries and parallel steer planataries; and

[0016]FIG. 9 is a block diagram of the transmission of FIG. 8.

DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

[0017] The vehicles intended for application of this transmission arework vehicles with high maneuverability including counter-rotation andspin turns. Many have a requirement for continuous forward to reversecycling. It is desirable to have a continuous ratio throughout thevehicle speed range in order to allow maximum flexibility for the driveror the work to be done. The transmission output drives are typicallylocated adjacent to the wheels or tracks and are close coupled to thewheel or track drive. This might be gears or chains depending on thevehicle needs. The engine may be positioned longitudinally with respectto the vehicle direction of travel, or transversely in order toaccommodate space or weight distribution needs. The maximum output speedmay vary according to the vehicle vocation. The maximum torquerequirement in reverse may be lower than in forward.

[0018] Hydromechanical transmissions are characterized by a hydrostatictransmission power path in parallel with a mechanical power transmissionpath, arranged in a manner to decrease the average power flow throughthe hydrostatic portion to thereby increase operating efficiency.Typically, the mechanical power path includes a planetary gear set whichacts to sum the power flows at either the input or output end of thetransmission.

[0019] The existence of parallel power paths creates the possibility ofreducing the output speed range or torque ratio in order to furtherreduce transmitted hydrostatic power. This then requires multiple rangesor “modes” to achieve the full torque and speed range of thetransmission. The impact of multiple modes is to improve efficiency andsometimes to reduce cost. In addition to efficiency and cost, themagnitude of the output speed range/torque ratio in each mode has animpact on input power capacity relative to the size of the HST. Smallerratios allow larger input power for the same size hydrostatic units. Itis obvious that more modes allow either smaller mode ratios or largertransmission ratios or both. These relationships create the possibilityfor having a versatile design configuration that accommodates a numberof market needs for input power, ratio range and efficiency.

[0020] Multi-mode HMT's are usually accomplished by reusing thehydrostatic components and clutching to a different mechanicalcomponent. The mechanical component will be a planetary if the mode ishydromechanical. Usually the modes are arranged so that there is noratio change during the mode change in order to have continuous speed ortorque delivery. Also, the hydrostatic transmission is usually strokedover center from full positive displacement to full negativedisplacement in order to fully utilize the installed hydrostatic power.

[0021] Differential steer transmissions have two inputs and two outputs.One input is for vehicle average speed and one is for steering, and eachoutput powers a side of the vehicle. The differential steering inputregulates the relative speed of each side of the vehicle, usually bysubtracting speed from one side and adding it to the other. Differentialsteer speed is usually powered by a hydrostatic transmission. Theforward/reverse speed input may be powered by any transmission form.

[0022] With reference to FIG. 1, the engine 1 crankshaft is positionedtransversely to the direction of vehicle motion. For compact vehicles,the transmission 71 is mounted parallel to the engine and is driven by abelt 77. For vehicles with allowable space, the transmission may bemounted directly to the engine. The transmission output shafts 16-1 and16-2, which are separately operator controlled, are connected to a drivetrain and to the wheels. In the case illustrated, shaft 16-1 drives gearset 75-1/75-2/75-3, which in turn drive wheels 73-1 and 73-2 at the samespeed. Shaft 16-2 drives gear set 76-1/76-2/76-3 and wheels 73-3 and73-4 at the same speed. Some vehicles might use a chain set or adifferent gear arrangement to drive the wheels.

[0023] With reference to FIG. 2, the engine 1 crankshaft is positionedparallel to the direction of vehicle motion. The transmission 71, whichhas an internal right angle drive, is mounted directly to the engine.The transmission outputs are connected to the wheels as in thetransverse example above.

[0024] Transmission average output speed and torque are shown in FIG.3A, and output speed vs. HST F-unit speed in FIG. 3B. Both 2-mode and3-mode HMT's are shown. The number of modes and the exact scale of thetorque and speed would be a result of the vehicle needs. All modes arehydromechanical and have a split power flow. The speed for both mode 1forward and reverse mode start at zero speed and are continuouslyincreasing in speed until the limit of the hydrostatic units is reached.This allows continuous cycling forward to reverse while maintainingcontinuous speed and torque control. As mode 1 forward and reverse modeare separate hydromechanical modes, maximum torque in reverse need notbe the same as maximum torque in forward. If a higher forward speed isrequired, mode 2 is added at the end of mode 1 and it is also continuousin ratio. The F-unit 37 (FIG. 3B) of the hydrostatic transmission iscontinuous in speed between modes and reaches full design speed at thebeginning and end of each mode.

[0025]FIG. 4 shows a schematic drawing for transmission circuitry andcomponents having a four element HMT planetary and coaxial clutches, andseries steer planetaries.

[0026] For the HMT portion, primary component groups are the hydrostatictransmission 51, 4-element planetary summer 49 which consists of ring80, ring 84, sun 82 and carrier 99, and three clutches 22, 23 and 24.Differential steer planetaries 46 and 50 are active when steering. Inthe start-up mode, which is hydromechanical, clutch 22 is engaged formode 1 forward that enables engine power to flow to sun 82. Engine 1 isconnected through shaft 38 to gear set 2/10 to the hydrostatictransmission 51. However as both V-unit 37 and F-unit 36 are at maximumdisplacement and the same speed, no power is being transmitted. As theoperator and programmed logic commands, a controller strokes V-unit 37displacement to a smaller value. Note that power is now being deliveredto planetary 49 through gear set 8/9 to ring 80, and through shaft 38 tosun 82, creating parallel power paths. Power is transmitted from bothpaths to planets 81/83-1, 81/83-2 and 81/83-3 to carrier 99, to gear set95/96 and to outputs 16-1, 16-2. Because ring 80 is speed controlled byHST 51, a variable speed is controlled at outputs 16-1, 16-2. As V-unit37 is stroked toward zero displacement, F-unit 36 slows and rotation ofgear set 9/8 also slows, which speeds up carrier 99 and outputs 16-1,16-2. As V-unit 37 is stroked through zero and then to maximum in thenegative direction, carrier 99 continues to speed up and the outputshafts 16-1 and 16-2 reach the maximum forward speed for mode 1. Thestroke control logic for the V-unit that resides in a controller may beof any type and may be like that described in U.S. Pat. No. 5,560,203.

[0027] For reverse direction of travel, clutch 23 is engaged. This isdone at zero output speed with ring 84 and gear 89 at the same nominalspeed, which is negative with respect to engine rotation. At thiscondition, V-unit 37 is fully stroked in a positive direction. Gear set2/14/15/20/89 is driven by the input shaft 38, enabling power flow inplanetary 49 through ring 84, and in ring 80 through HST 51, creating aparallel power path. As clutch 22 is disengaged, sun 82 turns freepreventing power flow. The controller strokes V-unit 37 from fullpositive to full negative displacement, first reducing the speed ofF-unit 36 to zero and then increasing it to full negative speed, whichcauses carrier 99 and outputs 16-1, 16-2 to increase in speed withreverse rotation. With a variable speed from F-unit 36 to regulate ring80 speed, and a fixed speed from input 38 to determine ring 84 speed,output speed is controlled between zero and its maximum value in reverseby V-unit 37. The stroke control logic for V-unit 37 is consistent withmode 1 forward.

[0028] If a second forward mode is required, a mode change is initiatedand clutch 22 and 24 are shifted. At the fully negative stroked positionof V-unit 37, ring 84 and ring 82 of planetary 49 are at the samenominal speed. When clutch 24 is engaged, power from input shaft 38 isdelivered to ring 84, and power is delivered to ring 80 through HST 51.Sun 82 turns free. The controller strokes V-unit 37 from full negativeto full positive displacement and output speed delivered through carrier99 and gear set 95/96 to shafts 16-1 and 16-2 and output speed reachesmaximum for mode 2 forward. The stroke control logic for V-unit 37 isconsistent with mode 1 forward and reverse mode.

[0029] Note that continuous power is delivered from the engine to thewheels, with continuous ratio change, from full reverse to full forwardspeed even though the transmission changes modes at zero speed and atabout half forward speed. The gear ratios may be different toaccommodate different torque/speed ratio spreads for the HMT.

[0030] Also, note that planetary 49 has four rotatable power elementsbut only one set of planet axes. This is accomplished by having a normalthree element planetary with compound planets and engaging an extraelement with the planet gears. See FIGS. 6A and 6B for more detail. Ring80, planets 81/83, sun 82 and carrier 99 form a compound planetary withthree elements and a ratio of negative 1:1 between ring 80 and sun 82. Anegative ratio is defined as having one element rotate opposite theother when the carrier is fixed. Adding ring 84 forms a fourth elementwith ring 84/sun 82 ratio different than 80/82 and also in a negativedirection.

[0031] The differential steer portion is done with HST 52 andplanetaries 50 and 46. Planetaries 50 and 46 are similar and have anegative 1:1 ratio between the rings 79 and 88. The rings 79-1 and 79-2are connected with the same ratio as 88-1 and 88-2 except that one ispositive and the other is negative. When speed is applied to carrier87-1 by HMT output 95/96, planetary 50 applies equal torque to rings79-1 and 88-1. The speed of output shaft 16-1 and 16-2 is determined bythe action of planetary 46. For straight-ahead motion, F-unit 98 is atzero speed which locks gear set 91/90 and carrier 87-2. With carrier87-2 locked, rings 79-2 and 88-2 are constrained to operate in theopposite direction but at the same speed. As these rings are alsoconnected with output shafts 16-1 and 16-2 with the same ratio butopposite rotation, both shafts 16 are constrained to operate at the samespeed and in the same direction, producing straight-line motion for thevehicle. When V-unit 97 is stroked in one direction, F-unit 98 turnsgear set 91/90 and rotates carrier 87-2. This requires rings 79-2 and88-2 to change speed in an amount and direction equal to the change incarrier speed. This then has the effect of adding speed to one of shafts16-1 or 16-2 and subtracting an equal amount from the other, producingsteering of the vehicle. Reversing the direction of HST 52 will reversethe direction of the differential speed at the output shafts. Note thatthe effect of this planetary arrangement is for the HMT input to controlaverage output speed and the HST input to control differential outputspeed.

[0032] Planetaries 50 and 46 have two rings 79 and 88, two planets 89and no sun gear. The planets act as reversing idler gears except thatthey are mounted on a rotatable member. See FIGS. 7A and 7B for a moredetailed drawing of planetaries 50 and 46. Ring 79 and 88 are the samesize and are mounted on the same centerline. Each ring meshes with oneof the planets separately. The two planets 89-1 and 89-2, which meshtogether, are mounted on carrier 87, which also rotates on ring 79/88centerline. If the carrier rotation is fixed, ring 79 rotates at thesame speed but opposite rotation of ring 88 (a negative 1:1 ratio).

[0033]FIG. 5 shows a block diagram for the transmission of FIG. 4 havinga four element HMT planetary and coaxial clutches, and series steerplanetaries. A two-or three-mode HMT is created by having HST 51 inparallel with two or three alternate mechanical power paths defined byeither clutch 22, 23 or 24. Clutch 23 has a speed reversing gear 14 toproduce reverse output speed. Four-element planetary 49 sums theparallel power flows and delivers a continuously variable speed andtorque to the output gear set 95/96. The differential steer is createdby planetaries 50 and 46 in series with HMT output 95/96 and HST 52. Theplanetaries have a speed reversing gear 93 on one power path connection.HST 52 controls the differential speed between output shaft 16-1 and16-2 by adding speed to one and subtracting speed from the other throughinteraction of the series planetary arrangement.

[0034]FIG. 9 is the block diagram of an HMT having parallel axis HMTplanetaries and parallel power flow planetaries which are used fordifferential steer. The 2- or 3-mode HMT is created by having HST 51 inparallel with two or three alternate mechanical power paths defined byeither clutch 25, 26 or 27. Clutch 27 has a speed reversing gear 67 toproduce reverse output speed. Four-element planetary 69/70, which iscreated by continuously connecting two elements from each three elementplanetary, sums the parallel power flows and delivers a continuouslyvariable speed and torque to the two output shafts 16. The differentialsteer is created by planetaries 53-1 and 53-2 in parallel with HMToutput 11/54 and HST 52. The planetaries have a speed reversing gear 62on one power path connection. HST 52 controls the differential speedbetween output shaft 16-1 and 162 by adding speed to one and subtractingspeed from the other of the parallel planetaries.

[0035] Shown schematically in FIG. 8, primary component groups arehydrostatic transmission 51, 3-element planetary summer 69, whichconsists of ring 3, sun 5 and carrier 6, 3-element planetary 70, whichconsists of ring 32, sun 34 and carrier 35, and three clutches 25, 26and 27. Planetaries 69 and 70 are interconnected at gear sets 19/18 and7/11 which forms four independent planetary elements on two separateaxes of rotation. Differential steer planeteries 53-1 and 53-2, andsteer hydrostatic 52 are active when steering. In the start-up mode,which is hydromechanical, clutch 25 is engaged for mode 1 forward thatenables engine power to flow to sun 5. Engine 1 is connected throughshaft 38 to gear set 64/65 and 66/10 to the hydrostatic transmission 51,however as both V-unit 37 and F-unit 36 are at maximum displacement andthe same speed, no power is being transmitted. As the operator andprogrammed logic commands, a controller strokes V-unit 37 displacementto a smaller value. Note that power is now being delivered toplanetaries 69/70 through sun 34, and through gear set 64/65 to sun 5,creating parallel power paths. Power is transmitted from both paths tocarrier 6, to gear set Jul. 11, 1954 and to outputs 16. Because sun 34is speed controlled by HST 51, a variable speed is controlled at outputs16. As V-unit 37 is stroked toward zero displacement, F-unit 36 slows,which speeds up carrier 6 and outputs 16. As V-unit 37 is stroked thoughzero and then to maximum in the negative direction, carrier 6 continuesto speed up and the output shafts 16-1 and 162 reach the maximum forwardspeed for mode 1. The stroke control logic for the V-unit that residesin a controller may be of any type and may be like that described inU.S. Pat. No. 5,560,203.

[0036] For reverse direction of travel, clutch 27 is engaged. This isdone at zero output speed with carrier 35 and gear 18 at the samenominal speed, which is negative with respect to input rotation. At thiscondition, V-unit 37 is fully stroked in a positive direction. Gear set64/65/66/67/68 is driven by the input shaft 38, enabling power flow inplanetary 70 through carrier 35, and in sun 34 through HST 51, creatinga parallel power path. As clutch 25 is disengaged, sun 5 turns freepreventing power flow in planetary 69. The controller strokes V-unit 37from full positive to full negative displacement, first reducing thespeed of F-unit 36 to zero and then increasing it to full negativespeed, which causes ring 32 and outputs 16 to increase in speed withreverse rotation. With a variable speed from F-unit 36 to regulate sun34 speed, and a fixed speed from input 38 to determine carrier 35 speed,output speed is controlled between zero and its maximum value inreverse. The stroke control logic for V-unit 37 is consistent with mode1 forward.

[0037] If a second forward mode is required, a mode change is initiatedand clutch 25 and 26 are shifted. At the fully negative stroked positionof V-unit 37, ring 3 and sun 5 of planetary 69 are at the same nominalspeed. When clutch 26 is engaged, power from input shaft 38 is deliveredto carrier 35, and power is delivered to sun 34 through HST 51. Thecontroller strokes V-unit 37 from full negative to full positivedisplacement and output speed delivered through ring 32 and gear set11/54 to shafts 16-1 and 16-2 and output speed reaches maximum for mode2 forward. The stroke control logic for V-unit 37 is consistent withmode 1 forward and reverse mode.

[0038] Note that continuous power is delivered from the engine to thewheels, with continuous ratio change, from full reverse to full forwardspeed even though the transmission changes modes at zero speed and atabout half forward speed. The gear ratios may be different toaccommodate different torque/speed ratio spreads for the HMT.

[0039] Planetary 69/70 (FIG. 8) has four independent power elements buton two axes of rotation. This is accomplished by having two normalthree-element planetaries and continuously engaging two elements of eachplanetary with gears. The ratio selected between the planetary axesallows the F-unit 36 to be direct connected to sun 34, which may beapproximately 3 times input speed. As the operating speed of clutch 27is about ⅓ times F-unit speed, this combination also allows normal speedand torque on clutch 27.

[0040] The differential steer is done with HST 52 and simple planetaries53-1 and 53-2. Planetaries 53-1 and 53-2 are similar and have a negativeratio between the rings 56 and suns 59. The rings 56-1 and 56-2 areconnected to F-unit 98 with the same ratio except one is positive andthe other is negative. When speed is applied to sun 59-1 and 59-2 by HMToutput 11/54, equal torque is also applied to carriers 58-1 and 58-2 asthe planetaries have the same ratio. The speed of output shaft 16-1 and16-2 is determined by the rotation of rings 56-1 and 56-2. Forstraight-ahead motion, F-unit 98 is at zero speed which locks gear set55/60 and 61/62/63 and also locks rings 56-1 and 56-2. With the ringslocked and suns interconnected, carriers 58-1 and 58-2 are constrainedto operate in the same direction and at the same speed, producingstraight-line motion for the vehicle. When V-unit 97 is stroked in onedirection, F-unit 98 turns gear set 55/60 and 61/62/63, and rotates ring56-1 and 56-2 at equal speed but in opposite directions. This then hasthe effect of adding speed to one of shafts 16-1 or 16-2 and subtractingan equal amount from the other, producing steering of the vehicle.Reversing the direction of HST 52 will reverse the direction of thedifferential speed at the output shafts. Note that the effect of thisplanetary arrangement is for the HMT input to control average outputspeed and the HST input to control differential output speed.

[0041] It is therefore seen that this invention will achieve at leastall of its stated objectives.

What is claimed is:
 1. A vehicle with at least two wheels on each sideof the machine, an engine, a transmission with two output shafts todrive each side of the vehicle, a hydromechanical portion of thetransmission which regulates the average vehicle speed, and ahydrostatic portion of the transmission which regulates the side-to-sidedifferential output shaft speed for purposes of steering the vehicle. 2.The vehicle of claim 1 wherein the wheel geometry of the vehicle isfixed.
 3. The vehicle of claim 1 wherein the transmission is mountedtransversely with respect to a longitudinal axis of the vehicle.
 4. Thevehicle of claim 1 wherein the transmission is mounted longitudinallywith respect to the vehicle motion.
 5. A transmission having an inputand two outputs, with a first portion controlling average output speedconsisting of a first HST in parallel with at least one planetary andtwo clutches, and a second portion controlling differential output speedand having a second HST, the first portion having a first and secondoperating mode.
 6. The transmission of claim 5 wherein the firstoperating mode starts at zero speed forward and the second operatingmode starts at zero speed reverse.
 7. The transmission of claim 6wherein a third operating mode is obtained with a third clutch and is ahigh-speed mode in sequence with the first mode.
 8. The transmission ofclaim 5 wherein the second portion produces added speed to one outputand a like amount of speed subtracted from the other output, where thedifferential speed is controlled by the second HST.
 9. An HMT with asingle, unitized four-element planetary system used as a power-summingmeans comprising a sun gear, two rings and a carrier, and a maximum ofthree planet gear centerlines.
 10. The HMT of claim 9 wherein clutchesare located on a single centerline and are co-axially positioned withthe four-element planetary.
 11. The HMT of claim 9 wherein the carrieris the output member of the four-element planetary.
 12. An HMT with afour-element planetary used as a power summing means comprising firstand second planetaries on two centerlines of rotation, each havingfirst, second and third gear elements, and drivingly connected togetherwith a gear ratio at two drive elements from each planetary.
 13. The HMTof claim 12 wherein the gear ratio is suitable to allow directconnection of a fixed hydraulic unit to the second planetary.
 14. TheHMT of claim 12 wherein a second input driven clutch is directlyconnected to a first gear element on the second planetary, and a firstinput driven clutch is directly connected to a first gear element of thefirst planetary.
 15. The HMT of claim 12 wherein a third input drivenclutch is directly connected to a second gear element of the firstplanetary which is also gear connected to the first element of thesecond planetary.
 16. The HMT of claim 12 wherein the carrier of thefirst planetary is an output element.
 17. The HMT of claim 8 wherein thesecond portion consists of second and third simple planetaries inparallel to the first portion and in parallel to the second HST, thespeed differential resulting from plus speed to one element of the firstplanetary and minus speed to the same element of the second planetarywhen the second HST is rotated.
 18. A differential speed transmissionhaving first and second inputs, an HST connected to the second input, afirst and second planetary each with first, second and third elements,the first planetary connected with the first input at the first element,and the second planetary is connected to the second input at the firstelement, the planetaries being connected to each other at the secondelement, and connected to each other at the third element with areversing drive, with the average output speed being controlled by thefirst input, and the differential output speed being controlled by theHST through the second input.
 19. The transmission of claim 18 whereinthe first element is a carrier, the second and third elements are rings,and each planetary has two planets which are drivingly connected to eachother and each is connected to a different ring.
 20. The transmission ofclaim 18 wherein the first input is from an HMT output.